1. Field of the Invention
The present invention relates to a production method for a semiconductor device employing a silicon carbide semiconductor substrate.
2. Description of Related Art
In a production process for a semiconductor device employing a silicon carbide (SiC) semiconductor substrate, an impurity region is formed in a surface of the silicon carbide semiconductor substrate by performing ion implantation and then annealing (heat treatment).
The annealing after the ion implantation is achieved, for example, by mounting the ion-implanted silicon carbide semiconductor substrate on a susceptor of graphite disposed in a quartz tube with the front surface (device formation surface) of the substrate facing up and, in this state, causing the susceptor to generate heat through high frequency induction heating by supplying high frequency power to a coil wound around the outer periphery of the quartz tube. The temperature of the susceptor is 1600 to 1800° C. during the annealing. Ions (impurity) implanted into the front surface of the silicon carbide semiconductor substrate are activated by the heat from the high temperature susceptor.
In the aforementioned annealing method, however, Si atoms in the front surface of the silicon carbide semiconductor substrate are sublimated into the ambient atmosphere. Further, Si atoms or C atoms migrate in the front surface of the silicon carbide semiconductor substrate. As a result, the crystalline structure of SiC is changed, so that the front surface of the silicon carbide semiconductor substrate is disadvantageously roughened.
In another prior-art method for annealing the silicon carbide semiconductor substrate, a cap of silicon carbide is brought into contact with the front surface of the silicon carbide semiconductor substrate mounted on the susceptor and, with the front surface of the silicon carbide semiconductor substrate covered with the cap, the silicon carbide semiconductor substrate is annealed. This method also has the possibility of the roughening of the front surface of the silicon carbide semiconductor substrate. That is, where the silicon carbide cap contacts the front surface of the silicon carbide semiconductor substrate, the sublimation of Si atoms occurs on a higher temperature side. Therefore, if the temperature of the silicon carbide semiconductor substrate is higher than the temperature of the silicon carbide cap, Si atoms in the front surface of the silicon carbide semiconductor substrate are sublimated, so that the SiC crystalline structure in the front surface of the substrate is changed. Further, Si atoms sublimated on the higher temperature side migrate to a lower temperature side. Therefore, if the temperature of the silicon carbide cap is higher than the temperature of the silicon carbide semiconductor substrate, Si atoms are sublimated from the silicon carbide cap, and the sublimated Si atoms adhere to the front surface of the silicon carbide semiconductor substrate. Therefore, the front surface of the silicon carbide semiconductor substrate is liable to be roughened, whichever of the silicon carbide semiconductor substrate and the silicon carbide cap has a higher temperature.
In a method proposed in Japanese Unexamined Patent Publication No. 2001-68428, the annealing is performed with a protective film formed on the front surface of the silicon carbide semiconductor substrate, whereby the roughening of the front surface of the silicon carbide semiconductor substrate and diffusion of impurity (boron) atoms from the front surface of the silicon carbide semiconductor substrate are prevented during the annealing. However, the proposed method is disadvantageous in that the protective film should be removed by plasma etching or the like after the annealing, increasing the number of production process steps and the production costs.